1. Field of the Invention
The present invention relates to a Ziegler-Natta catalyst, and particularly to a method for making the catalyst and a method for using the catalyst in a polymerization reaction.
2. Background of the Art
Ziegler-Natta catalysts are advantageously used in olefin polymerization reactions to produce stereoregulated linear polymers. Typically, such catalysts include a trialkyl aluminum (e.g., triethyl aluminum) in combination with a solid component which includes a transition metal compound such as titanium chloride as co-catalyst on a support including a magnesium halide, and Otypically a porous particulate support (e.g., silica, alumina, etc.), and optionally an internal electron donor. Generally, the Ziegler-Natta catalysts are small, solid particles, but soluble forms and supported catalysts have also been used.
Ziegler-Natta catalysts are especially useful for the homopolymerization and copolymerization ethylene, propylene, and other alk-1-enes to produce films, fibers and moldings.
U.S. Pat. Nos. 5,162,465 and 5,006,620 disclose the preparation of polymers of propene using a Ziegler-Natta catalyst system consisting of a titanium component which is based on a finely divided, shape-imparting silica gel and contains, in addition to titanium, magnesium, chlorine and a benzenecarboxylic acid derivative, an aluminum component and a silane component. Both patents have in common that the impregnation of the supporting particle is performed using magnesium organic precursor compounds. These compounds are, in general, air and moisture sensitive and require additional solvent addition to the catalyst preparation process. Secondly, the conversion of the magnesium organic precursor compound to the magnesium halide is conducted using halogenating agents like chlorine or hydrogen chloride, which are in general considered as hazardous and environmentally unfriendly compounds.
EP 288845 discloses a supported catalyst component comprising a magnesium halide and as support a particulate solid which solid is identified as silica gel and according to the preparation is derived from the reaction of said particulate support with a magnesium organic precursor compound. By using such a catalyst component it is possible to produce from alk-1-enes polymers having a good morphology and bulk density with a good catalytic efficiency.
EP-A 761696 relates to catalytic systems of the Ziegler-Natta type comprising as support particulate macroporous silica gels having a mean particle diameter of from 5 to 200 μm, which are formed from smaller sub-particles by spray-drying. Again, the precursor compound to the magnesium halide is taken from the class of organomagnesium compounds. The catalytic systems show a good productivity and stereospecifity in the polymerization of C2-C10 alk-1-enes.
EP 829490 B1 also describes a catalytic system of the Ziegler Natta-type where a particulate support is impregnated with a magnesium-organic compound and eventually converted to a magnesium compound containing halogen.
The above patents, which describe the prior art, have in common that the impregnation of the supporting particle is performed using magnesium organic precursor compounds. In particular EP 829490 B1 describes the conversion of supported organo-magnesium precursors to an intermediate containing magnesium-oxygen bonds. In subsequent steps the magnesium intermediate is converted to a halogen containing magnesium compound. However, the described procedure is rather unspecific with respect to the conversion step of the pre-cursor to the intermediate magnesium compound and yields catalysts with inferior performance, in particular the polymerization productivity.
It is therefore an object of the present invention to overcome the above-described limitations and to develop improved catalytic systems of the Ziegler-Natta-type, regarding the polymerization productivity. The resulting catalysts have a high activity and allow the production of polymers of α-alk-1-enes having a good morphology and bulk density.